They're collectable and valuable because of the association with famous players in the classic rock era, and the rarity of examples. A perfect copy of the Mona Lisa would be worth a few $, not a few milion $, even if it were indistinguishable to the viewer.

Many guitarists fetishize the 59 Les Paul because of the sound. The trick is that '59 Les Paul might not sound any different objectively, but the musician knows they are playing an iconic and rare instrument. This can make the musician play better than usual. Guitarists often attribute this to the "mojo" in the guitar; the trick is the mojo is in the player's belief.